As portable information devices such as a laptop computer and a cellular phone have become popular, a demand for electrochemical devices such as a primary battery, a secondary battery and an electric double layer capacitor used as power supplies for the devices has rapidly increased. It is particularly required for these electrochemical devices to be compact and light, and from into a thin film, and simultaneously improvement of reliability is also expected. In recent years, in addition to power supplies for portable information devices new applications such as power supplies for a hybrid electric car and energy storage have been developed, and thus have been required to further improve the reliability.
An electrolyte solution in which an electrolyte salt is dissolved in a solvent is generally used in an electrochemical device, and when leakage liquid and further an electrolyte solution are non-aqueous electrolyte solutions, troubles such as catching fire and setting fire are caused, which make a major factor in impairing the reliability. Accordingly, these problems can be solved by using a solid electrolyte in place of an electrolyte solution. Particularly, a polymer electrolyte is easy to form a thin film and has excellent mechanical properties and flexibility, and therefore is a highly promising material.
From such viewpoints, many investigations with respect to a polymer electrolyte have been made over the years and there have been many proposals since it was firstly reported that ionic conductivity was produced by composite formation of a certain kind of alkali metal salt with a poly(ethylene oxide)-based high polymer (see Non-Patent Document 1).
Patent Document 1 proposes semi-solid gel-type polymer electrolytes comprising methyl polymethacrylate, electrolyte salts such as LiClO4 or LiBF4, and organic solvent.
Patent Document 2 proposes an electrochemical generator using an all solid-type polymer electrolyte in which an electrolyte salt is solid-solubilized in a high polymer containing a heteroatom such as oxygen or nitrogen, and poly(ethylene oxide) and a polyamine, are shown as examples of a polymer material in the document.
Patent Document 3 proposes a gel-type polymer electrolyte composition in which an electrolyte salt is dissolved in a mixture of a high polymer having a dielectric constant of 4 or more and an organic solvent having a dielectric constant of 10 or more, and shows that examples of a polymer material satisfying such a requirement include nitrocellulose, a phenol resin, polyvinylidene fluoride, polyacrylonitrile and chlorosulfonated polyethylene.
Patent Document 4 discloses a lithium solid electrolyte cell using metal lithium as a negative electrode and metal chalcogenide as a positive electrode, and shows that examples of the solid electrolyte include polymer electrolytes using a vinylidene fluoride copolymer, polyvinyl chloride, polyvinyl acetate, polyvinyl pyrrolidone or the like.
Patent Document 5 proposes an ionic conductive solid composition using a polymer material and discloses polysiloxane as an excellent polymer material.
Patent Document 6 discloses a hybrid ion conductor using an oxyethylene (meth)acrylate polymer.
Further, Patent Document 7 discloses an ionic conductive crosslinking-type resin composition based on an aliphatic epoxy resin, Patent Document 8 discloses a polymer electrolyte based on polyphosphazene, Patent Document 9 discloses an ionic conductive polymer complex comprising polyalkylene carbonate, metal salts and organic solvent, Patent Document 10 discloses a polymer solid electrolyte and a polymer solid electrolyte cell using polyurethane, and Patent Document 11 discloses, for example, an ionic conductive composition based on polyvinyl alcohol.
As described above, with respect to a polymer electrolyte, two kinds of polymer materials of an all solid-type polymer electrolyte comprising a polymer material and an electrolyte salt and a gel-type polymer electrolyte mixed with a polymer material and an electrolyte salt, and further a solvent have been proposed, but the following significant problem still remains.
That is, no material achieving practically satisfying ionic conductivity was proposed for an all solid-type polymer electrolyte. Further, a large amount of solvent had to be mixed to obtain practical ionic conductivity in the case of a gel-type polymer electrolyte. Therefore, from the viewpoint of reliability, reliability of each of these electrolytes is only a level better than that of an electrochemical device using a conventional liquid electrolyte, and thus high reliability originally expected for a polymer electrolyte was not achieved.
Thereafter, keeping in line with commercialization of a lithium ion secondary battery, it was proposed to apply a polymer electrolyte to a lithium ion secondary battery (see Patent Document 12). Thereby, research of a polymer electrolyte has been actively conducted and a lithium ion secondary battery using a gel-type polymer electrolyte was commercialized. However, as described above, this gel-type polymer electrolyte contained a large amount of solvent, and high reliability originally expected for a polymer electrolyte was not obtained. As a result, in a lithium ion secondary battery market, most of the product is occupied by the one using a liquid electrolyte and the market share of a lithium ion secondary battery using a gel-type polymer electrolyte is extremely small. In order to solve this problem, various polymer materials have been investigated since then, and Patent Document 13 proposes an ionic conductive polymer electrolyte comprising a polymer A having a carbonyl group (1 to 40% by weight), a poly(vinylidene fluoride)-based polymer B (20 to 70% by weight), a metal salt C (1 to 50% by weight) and an organic solvent D (20 to 85% by weight). Herein, preferable examples of the polymer A having a carbonyl group include polyesters, polycarbonates and polyester carbonates, and the other examples thereof further include polyamides, polypeptides, polyurethanes and polyketones. However, this system also contains a large amount of organic solvent, and the ionic conductivity is not always satisfying.
As described above, although a lithium ion secondary battery using a gel-type polymer electrolyte is put into practical use for only partial application of compact consumer batteries, major problems still remain in development of a polymer electrolyte in the present situation.
Patent Document 1: JP-A-54-104541
Patent Document 2: JP-A-55-098480
Patent Document 3: JP-A-57-143356
Patent Document 4: JP-A-58-075779
Patent Document 5: JP-A-59-230058
Patent Document 6: JP-A-60-031555
Patent Document 7: JP-A-60-248724
Patent Document 8: JP-A-61-254626
Patent Document 9: JP-A-62-030147
Patent Document 10: JP-A-01-197974
Patent Document 11: JP-A-01-284508
Patent Document 12: JP-A-01-241767
Patent Document 13: JP-A-11-060870
Non-Patent Document 1: P. V. Wright, Polymer, 14, 589 (1973)